When starting a permanent magnet synchronous machine, knowledge of the magnet location contributes to maximizing starting torque and insuring that torque is produced in the desired direction.
The magnet orientation with respect to the applied electrical field determines both the magnitude and direction of developed torque on the rotor shaft. The developed torque is proportional to the cross product of the stator and rotor flux linkages as indicated in equation (1) below. The location of the stator flux linkage is controlled by the applied voltage to the stator, whereas the location of the rotor flux linkage is aligned with the rotor magnets. This relationship is illustrated in FIG. 1, which shows the rotor flux vector xcexr aligned with permanent magnets and the stator flux vector xcexs as determined by the stator voltage.
Temxe2x88x9dxcexsxc3x97xcexrxe2x80x83xe2x80x83(1)
Typically, the initial rotor position is either determined by using hall-effect sensors to detect the permanent magnet flux or set through a starting sequence to align the magnets (rotor) in a specified position. In many applications, it is desirable to detect the rotor position without the use of additional sensors while keeping an unlocked rotor at standstill. Not only is the elimination of the hall-effect sensors cost-effective, but overall system robustness can be increased because fewer electrical connections are required.
The magnitude of the permanent magnet rotor flux is an important parameter in torque production. One problem that occurs in torque production is demagnetization of a permanent magnet resulting in less rotor flux and consequently less torque production and loss of efficiency. Permanent magnet demagnetization can be uniform over the entire rotor structure or can be localized to a specific magnet, for example. It would be advantageous to know the health (magnetization quality) of the magnets before start up of the motor drive for diagnostic purposes.
The invention relates to a method including an algorithm for estimating the initial magnet (rotor electrical) position of a permanent magnet AC machine by injecting a small high frequency carrier signal. The advantage of this signal injection method is that it can work at any initial motor condition including standstill and rotating conditions. In addition, this method permits detection of demagnetization of the rotor magnets. The invention is achieved using phase tracking techniques that have the advantage of being less sensitive to parameter variation.
Initial position of the permanent magnet rotor contributes to a controlled start of the permanent magnet AC machine. The fundamental idea of the present invention is to estimate the rotor electrical position by measuring saturation of the stator caused by the permanent magnet field on the rotor. The saturation of the stator results in a parasitic inductance variation that corresponds to the magnet position.
A related utility of the present invention involves demagnetization of the motor permanent magnets that can occur over the lifetime of the machine and during fault conditions. The carrier signal injection technique according to the present invention permits detection of demagnetization.
An object of the present invention is to estimate the initial permanent magnet rotor position with respect to the stator orientation without using additional sensors, other than those required for current control of the drive. Another objective is to determine the magnetization of the PM machine before start up, and whether demagnetization has occurred.